SABnzbd is a multi-platform open source news client targeted at efficient retrieval of Usenet binary posts. It's written in Python and works on Windows, Mac OS X and POSIX-compliant systems. It tries to offer maximum automation in downloading and post processing.
Sick Beard is a PVR for newsgroup users (with limited torrent support). It watches for new episodes of your favorite shows and when they are posted it downloads them, sorts and renames them, and optionally generates metadata for them.
CouchPotato is an automatic NZB and torrent downloader. You can keep a "want to watch"-list and it will search for NZBs/torrents of these items every X hours. Once a correct release is found, matching the correct quality, it will send it to SABnzbd or download the .nzb or .torrent to a specified directory.
A great tutorial for installing and configuring these three applications can be found at Ainer.org under the Tutorials section. To summarise: